Add Jira troubleshooting on reverse proxy usage

What does this MR do and why?

As determined in this ticket and request for help, there are some situations when a reverse proxy is used in front of GitLab that can cause the Jira integration to behave unexpectedly. This MR adds additional documentation to reflect that the 'app key' should always match between Atlassian and GitLab, and references that the Failed to link group. Please try again error can also be a result of the app key being replaced.

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.